Process for quenching hot iron powder agglomerates



PROCESS FOR QUENCHING HOT IRON POWDER AGGLOMERATES Filgd 7 Nov. 15, 1957 INVENTORS 1460/56 4'. 46404444 Ina/4M 1. 04/45, J0. BY

PROCESS FOR QUENCHING HOT IRON POWDER AGGLOMERATES Jagdish C. Agarwal, Verona, Pa., and William L. Davis,

Jr., Chicago, Ill., assignors to United States Steel Corporation, a corporation of New Jersey Filed Nov. 15, 1957, Ser. No. 696,751

2 Claims. (Cl. 148-134) This invention relates to an improved method of quenching hot iron powder agglomerates. As used herein, the term iron powder agglomerates refers to agglomerates formed of iron oxide reduced wholly or partially to metallic iron, plus included gangue, and in which the oxide is reduced sulficiently (at least about 60 percent) that it has a strong tendency to reoxidize. The term also is intended to include equivalent metals, such as others of the iron group.

Fluidized bed processes for directly reducing iron oxide produce metallic iron in the form of impure powder initially at a temperature of at least 700 F. This powder reoxidizes violently until it has been agglomerated, and even then hot agglomerates reoxidize readily. Conventionally the powder is agglomerated by a hriquetting process or the equivalent while still at a temperature of atleast 700 F., and the agglomerates are cooled in an inert atmosphere, such as nitrogen. This cooling procedure has a disadvantage that it is unduly cumbersome and costly. Large volumes of nitrogen must be circulated around the agglomerates and recycled, requiring fans, blowers, heat exchangers, gas cleaners, conveyors and a screening station.

An object of our invention is to provide a simplified method of cooling hot iron powder agglomerates and preventing their reoxidation.

A further object is to provide a cooling method in which hot iron powder agglomerates are water quenched under carefully controlled critical conditions to produce a completely dry product not readily reoxidized.

A more specific object is to provide an improved cooling method in which hot iron powder agglomerates are immersed in water for only a few seconds and withdrawn at a temperature suflicient that excess sensible heat in the agglomerates evaporates all acquired moisture almost immediately, but leaves the agglomerates at a temperature insufficient to cause their reoxidation as they cool to ambient temperatures and are placed in bulk storage.

A further object is to provide an apparatus suitable for performing our improved cooling method and also facilitating recovery of unagglomerated iron powder.

In accomplishing these and other objects of the invention, we have provided improved details of structure, a preferred form of which is shown in the accompanying drawing, in which the single figure is a diagrammatic view of our apparatus.

The figure indicates a conventional agglomerating device 10, such as a roll-type briquetting machine, in which freshly reduced iron powder at a temperature of at least 700 F. can be agglomerated without reoxidation. Iron powder agglomerates discharge therefrom into a chute 12, the upper portion of which preferably contains a grizzly 13 and a partition 14 for intercepting and retuming residual fines. Agglomerates which remain on the grizzly fall through a side passage 15 in the chute into a quench tank 16. The lower end of the chute lies below the water level in the tank. Hence steam, which forms ice as hot agglomerates strike the water, rises in the chute and assists in preventing oxidation of agglomerates during their brief passage through the chute before they reach the water. The chute walls preferably contain one or more vents 17 through which excess steam can escape.

The receiving end of an upwardly sloping belt conveyor 18 is located within the quench tank 16 beneath the discharge from chute 12. The conveyor extends outwardly from the tank and discharges into any suitable receiver, such as a transfer car 19. The belt preferably is of woven wire mesh and carries transverse metal cleats 20 on its carrying surface to prevent material from falling back along its slope. A variable speed motor 21 continuously drives the conveyor.

Preferably the quench tank 16 is conical. A discharge line 22 is connected to the lower end of the tank and extends to the intake side of a continuously driven conventional centrifugal pump 23, which draws a water suspension of unagglomerated iron powder from the tank. A line 24 extends from the discharge side of the pump to a filter 25, which separatesiron powder for return to agglomerating device. filter is close to the boiling point, and preferably its sensible heat is utilized, for example in a hot water heating system, not shown. Line 24 contains a flow control valve 26 preferably equipped with an automatic operating device 27. Cold water is added to the quench tank through a supply line 28 which contains a flow control valve 29. Preferably this valve also is equipped with an automatic operating device 30.

Preferably our apparatus also has automatic control means 31 for the two valve operating devices 27 and 30. Per se these control means are conventional and not of our invention; hence no detailed showing is offered. Nevertheless one example of suitable control means is illustrated in a publication of the Foxboro' Company, Foxboro, Massachusetts, entitled General Catalog of Foxborolndustrial Instrumentation Bulletin 453, 1953, page 60, and identified therein as the Dual Controller variationof Model 40, Air Operated Controllers. A temperature connection 32 transmits a measurement of the water temperature in the quench tank to the control means, which acts on the valve operating device 30 to regulate the opening of valve 29, whereby the valve admits sufficient cool water through line 28 to maintain the desired water temperature in the tank. A pressure connection 33 transmits a measurement of the water level in the quench tank to the control means, which acts on the valve operating device 27 to regulate the opening of valve 26, whereby the valve allows pump 23 to withdraw sufiicient liquid from the tank to maintain the desired level. Preferably the pump has a bypass 34 to return excess liquid which cannot pass through valve 26.

According to our cooling method, We maintain water in the quench tank 16 at a temperature approaching the boiling point. Hot iron powder agglomerates continually feed into the water and of course quickly heat it to such a temperature. Controlled additions of cooler water through supply line 28 maintain the water temperature at this value and prevent the water from boiling away.

At the same time controlled discharge through line22 maintains water in the tank at the proper level just above the discharge end of chute 12. We immerse agglomerates in water at this temperature for a sufiicient'interval that the agglomerates are at a finishing temperature in the range of about 200 to 300 F. a few seconds after withdrawal from the water. We can control this interval by regulating the rate of travel of conveyor 18 through its variable speed drive motor 21. The immersion time needed to attain the desired finishing temperature varies principally with the size of the individual agglomerates,

Patented Mar. 14, 1961 Water discharged from the but other factors such as porosity and chemical composition have a bearing. The extreme range of immersion time is about 3 to 120 seconds, and the preferred about to 60 seconds. i

The finishing temperature of the agglomerates is highly critical. The temperature at which the agglomerates are withdrawn from the water must be sutficient that their sensible heat evaporates all acquired moisture almost immediately, and yet after drying they must besufliciently cool not to reoxidize. Dryness is essential since prolonged exposure to water inherently is oxidizing to iron; if the agglomerates remain moist, they reoxidize in bulk storage over an extended period with liberation of heat which is not readily dissipated. The results attained by our method were quite unexpected to us. We would normally expect some degree of reoxidation as agglomerates enter water, but chemical analysis proves there is none. We would also expect sudden chilling to create small fractures in the agglomerates and thus cause them to be brittle and break easily during handling. However, tumbling tests demonstrated that their strength is equal to that of nitrogen-cooled agglomerates.

A further advantage which our invention attains is that unagglomerated powder is almost completely recovered for agglomeration. Such powdersettles in the quench tank and is recovered completely as the discharge is filtered.

As a specific example, we produced iron powder briquettes in a roll-type briquetting machine, each weighing about 2 ounces and having dimension of about 1% x 1% x inches. The specific gravity of the briquettes was about 4, their iron content about 92 percent, and their initial temperature range 800 to 1100 F. as they left the briquetting operation. We immersed these relatively small briquettes in water at about 200 F. for 3 to 8 seconds, after which we withdrew them at 350 to 450 F. In to 30 seconds they evaporated themselves to dryness at a finishing temperature in the range 200 to 300 F. They were stable in bulk storage exposed to the atmosphere during final cooling to an ambient temperature of about 70 F.

From the foregoing description it is seen that we have provided a simple method and apparatus for cooling hot iron powder agglomerates. The method completely prevents reoxidation as effectively as cooling in an inert atmosphere, yet saves time and utilizes far less equipment.

While we have shown and described only a single embodiment of the invention, it is apparent that modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. A method of cooling hot iron powder agglomerates from an initial temperature of at least 700 F. and preventing their reoxidation comprising continuously immersing newly formed agglomerates in a body of water,

sensing the temperature of said body, adding cool water to said body in amounts governed by the temperature thus sensed to maintain said body at a temperature approaching the boiling point, sensing the level of water in said body, discharging a suspension of unagglomerated powder from the bottom of said body in amounts governed by the level thus sensed to maintain a predetermined level, continuously withdrawing said agglomerates from the water after an immersion time 'of 3 to seconds, employing sensible heat in the agglomerates to evaporate immediately all acquired moisture leaving the agglomerates at a finishing temperature in the range of about 200 to 300 F., and thereafter cooling said agglomerates to ambient temperatures.

2. A method as defined in claim 1 wherein said sus pension is filtered for recovery of iron powder and the recovered powder is agglomerated.

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1. A METHOD OF COOLING HOT IRON POWDER AGGLOMERATES FROM AN INITIAL TEMPERATURE OF AT LEAST 700*F. AND PREVENTING THEIR REOXIDATION COMPRISING CONTINUOUSLY IMMERSING NEWLY FORMED AGGLOMERATES IN A BODY OF WATER SENSING THE TEMPERATURE OF SAID BODY, ADDING COOL WATER, TO SAID BODY IN AMOUNTS GOVERNED BY THE TEMPERATURE THUS SENSED TO MAINTAIN SAID BODY AT A TEMPERATURE APPROACHING THE BOILING POINT, SENSING THE LEVEL OF WATER IN SAID BODY, DISCHARGING A SUSPENSION OF UNAGGLOMERATED POWDER FROM THE BOTTOM OF SAID BODY IN AMOUNTS GOVERNED BY THE LEVEL THUS SENSED TO MAINTAIN A PREDETERMINED LEVEL, CONTINUOUSLY WITHDRAWING SAID AGGLOMERATES FROM THE WATER AFTER AN IMMERSION TIME OF 3 TO 120 SECONDS, EMPLOYING SENSIBLE HEAT IN THE AGGLOMERATES TO EVAPORATE IMMEDIATELY ALL ACQUIRED MOISTURE LEAVING THE 